FIG. 1 shows a block diagram of an access control system described in Japanese Patent Application No. 101,439/1985 filed by the assignee of the present application on May 15, 1985 for an "Optical Disk Drive Device".
As illustrated, an optical disk 101 has a plurality of recording tracks on the disk. The tracks comprise series of pits disposed at a high density and arranged in a circular or spiral form. The disk 101 is fitted onto a spindle and is rotated by a disk-drive motor 102. The disk-drive motor 102 is rotated under the control of a disk-motor-drive control system 103.
An optical head 104 forms a light spot on the optical disk 101. The light spot is moved in the radial direction of the optical disk 101. The optical head 104 comprises a frame 105, a source of light such as a semiconductor laser 106, a collimating lens 107, a polarization beam splitter 108, a .lambda./4 plate 109, an optical-path changing mirror 110, an objective lens 111 which focuses the light beam from the light source 106 onto the medium surface of the optical disk 101 and forms a light spot 115 on the surface, a tracking actuator 112 which provides fine or microscopic movements of the objective lens 111 in the radial direction of the optical disk 101 for accurate positioning of the light spot on a recording track of the optical disk, and a split-photodetector 113 which has a pair of sensor parts adjacent to each other for detecting the return light reflected from the optical disk 101, and produces a pair of electrical signals corresponding to the amount of light received at the respective sensor parts.
An addition/subtraction amplifying circuit 114 determines the sum of the outputs from the split-photodetector 113 to produce a sum signal as an information signal (reproduced data signal), and determines the difference between the outputs of the split photodetector 113 to produce a difference signal as a tracking error signal. The tracking error signal is supplied to a track-traverse counter 118 and a speed-detecting circuit 120.
The track-traversing counter 118 receives the output signal from the addition/subtraction amplifying circuit 114 and detects the number of tracks traversed by the optical head 104. An output of this counter 118 is supplied to a target-velocity generation circuit 119.
The target-velocity generation circuit 119 receives the output signal of the track-traversing counter 118, and, at the time of access, generates a target-velocity signal for the light spot 115. The target-velocity signal is sent to a head-actuator drive control circuit 117.
The head-actuator drive control circuit 117 also receives an output signal from a polarity switching circuit 2. On the basis of these signals, the head actuator drive control circuit 117 controls the drive of a head actuator 116, such as a linear actuator.
When the head actuator 116 is driven through the head-actuator drive control circuit 117 to move the optical head 104 in the radial direction of the optical disk 101.
A speed detection circuit 120 detects the track-traverse speed (the speed with which light spot 115 traverses the tracks on the optical disk 115). The output of the speed detection circuit 120 is fed to the polarity switching circuit 2. The speed detection circuit 120, together with the polarity-switching circuit 2, forms a velocity-detection circuit 124.
The polarity-switching circuit 2 receives an output signal from an access-direction command generation circuit 123. Under the control of the output signal of the access-direction command generation circuit 123, the polarity-switching circuit 2 changes the polarity of the output of the speed detection circuit 120. More specifically, the detected speed (a scalar value) is converted into a detected velocity (a vector value) which also shows the direction.
FIG. 2 is a transfer-function block diagram of the velocity-control system which represents the diagram of FIG. 1. In this drawing, the results of the subtraction between the output signal V.sub.S * of the velocity detection circuit 124 and the output signal V.sub.r of the target-velocity generation circuit 119 is input to a gain compensation circuit 5. The gain compensation circuit 5 determines the frequency band of the velocity control system.
The gain compensation circuit 5, as well as a notch filter 122 and a head-actuator drive circuit 6, are built into the head-actuator drive control circuit 117. The notch filter 122 compensates the mechanical resonance characteristics G.sub.L (S) of a block 6 in the head actuator 116.
The head-actuator drive circuit 6 is normally of a current drive type, and also contains a drive current detection circuit.
A block 7 in the head actuator 116 represents a force constant of the head actuator 116. The block 8 represents transfer characteristics. Its input is an acceleration, and its output is a head velocity V.sub.L (the velocity with which the optical head 105 is moved by the head actuator 116). M designates the mass of the movable part, G.sub.L (S) designates the mechanical resonance characteristics of the head actuator, and S represents Laplacean. K.sub.V represents the sensitivity in the velocity detection of the target-velocity generation circuit 119 and the velocity-detection circuit 124, and .tau. represents the track traverse period (period taken for the light spot 115 to traverse a track).
FIG. 3 shows an example of gain characteristic of G.sub.L (S) in the block 8 in FIG. 2 which is shown to have a large resonance peak at a certain frequency .omega..sub.L (usually in the order of kHz). FIG. 4 on the other hand illustrates the gain characteristics .vertline.G.sub.N (S).vertline. of the notch filter 122 in FIG. 2. G.sub.n (S) is selected so that: EQU .vertline.G.sub.N (S).vertline..about..vertline.1/G.sub.L (S).vertline.,
when EQU .omega..sub.N =.omega..sub.L.
FIG. 5 and FIG. 6 show open-loop characteristics of the system of FIG. 2. FIG. 5 shows a case where G.sub.N (S)=1/G.sub.L (S), while FIG. 6 shows a case where G.sub.N (S).noteq.1/G.sub.L (S).
The system operates in the following manner:
First, the disk drive motor 102 is energized through the disk-drive-motor control circuit 103, and the optical disk 101 shown in FIG. 1 begins to rotate. When the rotation speed reaches a predetermined steady value, the tracking actuator 112 is controlled on the basis of a tracking error signal obtained by the photodetector 113 and addition/subtraction amplifying circuit 114. As a result, the light spot 115 begins to follow the center of a track on the optical disk 101.
At the time of track access, the number of tracks on the optical disk 101, which have been traversed by the light spot 115, is counted by the track-traversing counter 118, and, at the same time, in accordance with the number of tracks to be traversed to reach the target track, the target velocity, which is output from the target-velocity generation circuit 119, and the track-traverse velocity of the light spot 115, which is detected by the velocity-detection circuit 124, are input to the head-actuator drive control circuit 117, which performs velocity control in which the velocity is reduced to zero as the light spot 115 approaches the target track.
Operation of the velocity control system of FIG. 2 at the time of track access will now be described in detail. A velocity deviation signal Ve, which is the difference between the target-velocity signal V.sub.r from the target-velocity generation circuit 119 and the detected-velocity signal V.sub.s * from the velocity detector 124, is transmitted to the actuator drive circuit 6 through the gain-compensation circuit 5 and the notch filter 122. As a result, a certain drive current is applied to the head actuator 116. Due to this drive current, the head actuator 116 begins to move the optical head 104, causing the light spot 115 to traverse the tracks on the optical disk 101.
If the track fluctuation velocity, due for example to eccentricity of the optical disk 101, is denoted by Vd, the difference between the velocity of the head actuator 116 and the track fluctuation velocity V.sub.O is detected by the velocity detection circuit 124 as a detected-velocity signal V.sub.S *. This detected-velocity signal V.sub.S * is fed back in a velocity control system and the control is so made that the detected-velocity signal V.sub.S * coincides with the target-velocity signal V.sub.r.
A loop transfer function (open-loop characteristics) of this velocity-control system from the velocity-deviation signal Ve to the detected-velocity signal V.sub.S * can be expressed as follows: ##EQU1##
If it is so designed that the condition G.sub.N (S)=1/G.sub.L (S) is satisfied, the head actuator does not have the resonance frequency in the high-frequency zone, as shown in the upper part of FIG. 5. But if G.sub.N (S).noteq.1/G.sub.L (S) because of manufacturing fluctuations between individual devices, the head actuator may have a resonance frequency in the high-frequency band as shown in FIG. 6. When the peak of this resonance exceeds O.sub.db, the velocity-control system loses its stability.
Moreover, because of a certain dead time of the zero-order hold characteristics of the velocity-detection circuit 124, a long delay in phase is observed in the vicinity of the track-traversing frequency.
Because of the configuration described above, the conventional optical disk drive device had the following problems:
(1) The track traverse velocity is detected based on the track traverse period. When the traverse is made at a low velocity, a time delay (dead time) is lengthened, and the velocity control system loses its stability, and because the cut-off frequency of the velocity-control system must be designed low, the velocity deviation will be increased.
(2) When the light spot 115 traverses drop-out or data address recording portions, such traverse may erroneously be recognized as traverse of tracks, and, in spite of the slow movement of the light spot, the velocity detection circuit 124 erroneously operates as if the track traverse velocity were high. The result is that the velocity control system is disturbed.
(3) The head actuator 116 typically has a high mechanical resonance at the frequency of several kHz. In order to eliminate this phenomenon, a notch filter 122 is built in the head actuator drive-control system. If, however, there are a plurality of resonance frequencies, a plurality of notch filters need to be provided, and the size of the circuit of the system is therefore enlarged. In addition, where there are differences in the resonance frequency from one device to another, and the resonance frequencies differ from the frequencies of the notch filters, and the velocity-control system is not stable.
(4) Because the known system does not detect the direction in which light spot 115 traverses the tracks, but simply assumes that the light spot 115 is traversing the tracks in the direction (toward the inner or outer periphery of the disk) in which the access is to be made and determines the required velocity by changing the polarity of the speed. When the speed is low, and the direction in which the track is traversed is reversed due to track fluctuation or disturbances, a positive feed-back is applied to the system, and the optical disk may behave erratically.
Another problem of the above-described optical disk drive device is described below:
As the above-described optical disk drive device obtains head position control information from the optical disk, the absolute position of the head cannot be reliably detected when operation of the servo-system is disturbed, e. g., in case of application of a large external impact force. In the case, as well as in the case of abnormal operation of the servo-system the head may run out of the proper range toward the center or periphery of the disk and collide with a stopper located at the end of the range of mobility. As a result of this collision, the head receives a blow and can be broken.
The same problem relates to magnetic disk devices, or any other equivalent information storage devices.
Another example of prior art is explained below with reference to FIGS. 7 and 8.
FIG. 7 illustrates a block diagram which shows a control system of a known optical disk drive device published in Papers from the General Meeting of the Institute of Electronics and Communications Engineers (IECE) of Japan, 1985, Vol. 7, pp. 7-76 [1170, "Track Access in a Two-Stage Servo-System", by Hiroshi Inada and Shigeru Shimono]. FIG. 8 illustrates waveforms of control signals used in connection with the device shown in the block diagram. In these drawings, reference numeral 201 designates an optical disk for recording information, or with information already recorded on tracks which are arranged in the form of equally-spaced concentric circles or in the form of a spiral. Reference numeral 202 designates a light beam by means of which information is transferred to and from the optical disk. A head actuator, e.g., a linear actuator 205 drives a carriage 204 of an optical head 203 and moves the carriage 204 with respect to the optical disk 201 and across the tracks. A tracking actuator 206 is installed on the carriage 204 and carries a focusing lens for the formation of a spot of light beam 202 on the tracks of the optical disk 201. The tracking actuator 206 is moved in the same direction as the linear actuator 205 and can cover only a relatively small, predetermined number of tracks. A split photodector 207 which detects the information signal transmitted by the optical beam 202 and converts it into an electrical signal and outputs the electrical signal. A sensor of this detector consists of two parts. Each such part of the sensor produces on its output an electric signal corresponding to the quantity of light of the light beam 202 which is incident on this part.
A subtraction amplifier 211 receives a signal from each sensor part of the split photodetector 207, performs subtraction, and thus detects deviation of light spot of beam 202 from the center of the track on optical disk 201. A velocity detection circuit 212 detects, on the basis of an output signal from the subtraction amplifier 211, the track traverse velocity (the velocity with which the light spot of beam 202 traverses the tracks of optical disk 201 in its movement across the disk). A pulse generation circuit 213 receives signals from the subtraction amplifier 211 and generates a pulse each time the light spot of the beam 202 crosses a track on the disk. A track counter 214 receives a signal corresponding to the track access number N (the number of the tracks that must be traversed to reach the target track from the initial (currently-positioned) track) supplied from outside. The track counter 214 receives pulse signals from the pulse generation circuit 213 and counts down by "1" each time a pulse is applied to it, and its count value is the remaining tracks to be traversed to reach the target track. A reference velocity generation circuit 215 receives from the track counter 214 a signal corresponding to the remaining number of tracks, initially determines the reference velocity pattern corresponding to the number of the remaining track, memorizes this pattern, and then sequentially produces on its output the reference velocity signals corresponding to gradual decrease in the number of remaining tracks counted by the counter 214. A velocity error detector 216 receives a reference velocity signal from the reference velocity generation circuit 215 and a light spot velocity signal from the velocity detection circuit 212, and which detects the difference in velocities. An amplifying circuit 217 amplifies an output signal of the velocity error detection circuit 216 and controls the linear actuator 205. A position control command circuit 218 receives signals from the operational amplifier 211, the velocity control circuit 212, and the track counter 214. When on a predetermined track the velocity of the light spot of beam 202 drops below a predetermined value, the position control command circuit 218 produces a position control command on its output. A tracking servo-circuit 219 receives a position control command from the position control command circuit 218 and thus controls operation of the tracking actuator 206.
The above-described conventional optical disk drive device operates as follows: Track-access control is comprised of a velocity control mode and a position control mode. In the velocity control mode, the carriage 204 is driven by the linear actuator 205, to cause movement of the light spot in the direction of traverse of the tracks of the optical disk 201. In the position control mode, after the velocity of the light spot of the light beam 202 has been reduced below a predetermined velocity at the predetermined track, the tracking actuator 206 is controlled and the light spot is stopped at the position where the spot coincides with the center of a track on the disk 201 (FIGS. 8A-8C). First, in the velocity control mode, a signal which corresponds to track access number supplied from outside (number N in FIG. 7) is sent to the track counter 214. Because at the very beginning there are no pulses from the pulse generation circuit 213, so the number of the remaining tracks is left unchanged and the generated signal corresponds to this particular number N. Receiving this signal, the reference velocity generation circuit 215 initially determines the reference velocity pattern (FIG. 8A), and then sequentially outputs reference velocity signals in accordance with the number of remaining tracks as counted by the track counter 214. The reference velocity signal and the light spot velocity signal, which is produced by the velocity detection circuit 212, are input to the velocity error detection circuit 216 where both signals (i.e., of detected and reference velocities) are compared. The difference is amplified by the amplifier 217, and the amplified signal is used to control the velocity of the linear actuator 205. In accordance with the reference velocity pattern, the linear actuator 205 makes acceleration up to a predetermined number of tracks, the velocity is then stabilized until a predetermined number of tracks is reached, when the deceleration is made.
In this way, the light spot of beam 202 moves across the optical disk to reach the target track. When the light spot of beam 202 traverses a track, the quantity of light reflected from the optical disk 1 will change. As the sensor of photodetector 207 consists of two parts, the quantity of light reflected onto each sensor part also will vary. The light reflected onto the sensor is converted to electrical signals which correspond to the amount of light received by the sensor and which are output from the sensor parts. The output signals from the sensor parts are input to the subtraction amplifier 211 performs subtraction to produce a difference signal as shown in FIG. 8D. In this difference signal waveform, a zero point of each cycle corresponds to the moment when the center of the track on optical disk 1 coincides with the center of the light spot of beam 202. The velocity detection circuit 212 receives the output difference signals from the subtraction amplifier 211 and detects on the basis of these signals the track traverse velocity. The pulse generation circuit 213 generates pulses, for example, at the moment of each cycle when the difference signal waveform of the output from the subtraction amplifier 211 passes through zero. Each such pulse is used as a signal indicating that the light spot of beam 202 crossed the track. The pulses are supplied to the track counter 214. The position control command circuit 218 receives output signals of the subtraction amplifier 211, the velocity detection circuit 212, and the track counter 214. If at the moment of arrival of the light spot at a position with a predetermined number of tracks to the target track, e.g., one track to the target one, the velocity is below a predetermined value, the position control command circuit 218 will issue an output command which will switch the system to the position control mode.
In the position control mode, the tracking servo-circuit 219 receives the output signals of the position control command circuit 218 and the subtraction amplifier 211, and controls the tracking actuator 206 referring to the phase of the difference signal waveform from the subtraction amplifier 211. When the center of the target track of the optical disk 201 coincides with the center of the light spot of beam 202, the tracking actuator stops. Thus, pull-in into the track is completed. The light spot of beam 202 follows the target track, and recording and reproduction of information is conducted.
In the known optical disk drive device of the type described above, the carriage 204 is driven by the linear actuator 205, and when the target track is reached and operation of the linear actuator 205 and tracking actuator 206 is switched from the velocity control mode to the tracking control mode, the detected speed may be disturbed either by defects in optical disk 201, or by sudden deviation in the actual velocity due for example to external forces. As a result, the pull-in by the tracking servo-circuit 219 may not be achieved. In such a case, the system may behave erratically, unless an external position or velocity scale is provided.